1. Field of the Invention
The present invention relates in general to a brake pedal apparatus, and more particularly to an improvement of a brake pedal apparatus in which a characteristic of a lever ratio is changed depending upon a depression force.
2. Discussion of Prior Art
There is known a brake pedal apparatus having (a) an operating pedal which is disposed pivotably about a first axis, (b) a pivot member which is disposed pivotably about a second axis parallel to the first axis, and which is connected to the operating pedal via a connecting link so as to be mechanically pivoted about the second axis in response to a depressing operation effected on the operating pedal; and (c) an output member which is to be mechanically pressed or pulled as a result of pivot motion of the pivot member. As an example of such a brake pedal apparatus, Patent Document 1 discloses an apparatus in which a lever ratio representative of a ratio of an output to a depression force acting on the operating pedal is changed in accordance with a pedal stroke, and a lever ratio characteristic, i.e., a characteristic of the lever ratio in relation with change of the pedal stroke is dependent on connected positions of the connecting link.
Meanwhile, Patent Document 2 discloses an apparatus in which the output member (push rod) is connected directly to the operating pedal. When a depression force exceeds a predetermined value, a connected position at which the output member is connected to the operating pedal is changed against a biasing force of a spring, such that a lever ratio is increased. That is, a larger braking force can be generated when a large depression force is applied, for example, in the event of emergency braking.
Patent Document 1: JP-H7-205776A
Patent Document 2: JP-H5-301565A
However, in Patent Document 1, the lever ratio is changed merely in accordance with the constant lever ratio characteristic which is dependent on the connected positions of the connecting link. Therefore, when the brake pedal apparatus is operated with application of a large depression force thereto for quickly obtaining a large braking force, for example, it is not necessarily possible to obtain a braking force and a braking operation feeling corresponding to a braking command given by a vehicle driver. In Patent Document 2, it is possible to obtain an improved braking operation feeling owing to the arrangement in which the lever ratio characteristic is changed in accordance with the depression force exerted by a vehicle driver. In Patent Document 2, however, the change of the connected position, at which the output member is connected to the operating pedal, results in a change of posture (angle) of the output member, thereby providing a limitation in amount of possible displacement of the connected position and consequently making it difficult to largely change the lever ratio characteristic. Further, in Patent Document 2, the connected position is directly biased by a spring, which is required to generate a large spring force and accordingly has to be large in size and high in cost.
The present invention was made in the light of the background art discussed above. It is therefore an object of the invention to make it possible to make a brake pedal apparatus compact in size at a reduced cost, which apparatus is capable of providing a satisfactory braking operation feeling by largely changing its lever ratio characteristic in accordance with a braking command (depression force) given by a vehicle driver.